Frequency divider circuits



Oct. 4, 1949. w. J. OBRIEN FREQUENCY DIV'VIDER CIRCUITS Filea Aug.271945 www A'A'A'A'A'A'A'AVA'AVAVA' III gIB---lll' IN VEN TOR.

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'1 claims. (c1. 25o-36) My invention relates to frequency dividercircuits and has particular reference to thermionic vacuum tubelcircuits for converting an input signal of given frequency to vanoutput signal of lower frequency, and having a frequency dividing rangewhich is much larger than that of previous circuits designed for similaruse.

In the radio transmission and control art, it is often necessary toproduce a frequency which is less than that of a given source of radiofrequency. This is usually accomplished by means of multi-vibratorcircuits.

Multi-vibrator circuits have the disadvantage of producing a change inoutput frequency with changes in input voltage exceeding ten percent.Also the frequency of the output of a multi-vibrator circuit iscontrolled by the charging and discharging time of a condenser in aresistance capacity circuit. Since this time is a function of thevoltage applied to the condenser, the multi-vibrator circuit issensitive to voltage changes and does not necessarily produce an outputfrequency which bears the desired relationship to the input frequency.

It is, therefore, an object of my invention to provide frequencydividing circuits in which the output frequency is substantiallyindependent of the input voltage.

It is also an object of my invention to provide a frequency dividercircuit of the character set forth in the preceding paragraph which maybe used at either audio or radio frequency.

It is an additional object of my invention to provide a frequencydividing circuit in which the output frequency is determined by a tunedcircuit to make the output frequency substantially independent of theinput voltage.

It is a still further object of my invention to provide in a frequencydividing circuit of the character described a synchronizing circuit forsynchronizing the output frequency with the input frequency so that thetwo frequencies bear a whole number ratio to each other.

It is additionally an object of my invention to provide a frequencydivider circuit of the character set forth in which the output signal ischaracterized by including a number of harmonies to facilitate the useof a plurality of subsequent frequency multiplying circuits.

Other objects and advantages of my invention will be apparent from astudy of the following specifications, read in connection with theaccompanying drawings, wherein:

Fig. l is a schematic wiring diagram illustrating a frequency dividercircuit intended particularly for use at radio frequencies; and

Fig. 2 is a schematic wiring diagram illustrating a frequency dividercircuit which is intended particularly for use at audio frequency.

Referring to the drawings, I have illustrated in Fig. l a frequencydividing circuit which is intended particularly for use at radiofrequencies. Infig. l the cathode heater circuits have been omitted asthese circuits are conventional and well understood. Similarly thesources of high voltage direct current used for plate and screen gridsupply have been omitted and connections to a suitable source of suchvoltage are indicated 'merely by anV arrow bearing the legend B+. It isto be understood that any suitable source of plate supply potential maybe used.

The circuit which is shown in Fig. 1 includes an input tube I and afrequency regulating tube 2. These vacuum tubes are shown as pentodetubes, although tetrode or screen grid type may be used. The radiofrequency input, the frequency of which is to be divided, is connectedto input terminals 3` and 4 which are connected to a primary winding 5is tuned to the input frequency by a condenser 'l connected across theprimary 5. The transformer 6 includes a secondary winding 8, one end ofwhich is connected as by conductor 9 to the grid of the vacuum tube I.The other end ofthe secondary 8 is connected to ground through aresistance Il! which is shunted by a condenser Il. The secondary 8 istuned to the input frequency by a condenser I2 connected across thewinding 8.

The cathode of the tube I is connected to ground through a cathode biasresistor i3 which is preferably by-passed by a condenser I4. Thesuppressor grid of the tube I is connected directly tothe cathode as bya conductor l5. The screen grid voltage for the tube I is obtained froma suitable source of high voltage direct current, to the positiveterminal of which is connected` a conductor I5. The conductor E6isconnected to the screen grid of the tube I through resistances II andIla. The mid-point between resistors I'I and Ila is connected to thecathode through a resistance I8.

The plate of the tube I is connected by a conductor i9 to the primary 20of an output transformer 2l. Y The other end of the primary winding 2l!is connected to the source of plate supply potential by a conductor 22.The primary winding 2li is shunted by a condenser 23 and a resistance24. The condenser 23 is used to tune the winding 20 to the frequencydesired for the output, the output signal appearing across theterminalsp25 and 2S connected to a secondary winding 21 of thetransformer 2l. The resistance 24 is connected across the tuned primary2li to reduce the Q of the circuit to the tube to makeV theoutputcircuit tune rather broadly. The -output frequency is so chosen that theratio between the input frequency and the output frequency is som'ewhole number such as, for example, three.

The conductor .IS which is .connected to the Vplate of the tube. l isalso coupled to the control grid of the tube 2 through a condenser 28.

' rfhe grid of the tube 2 is also connected to ground through aresistance 29. The cathode of the tube 2 is also connected to groundthrough a cathode biasing resistor 35 which is by-passed as by acondenser 3l. A conductor 32 serves to connect the suppressor griddirectly to the cathode.

The plate of the tube 2 is connected by a conductor 33 -to a tuning coilor inductance 34. The other terminal is connected as by means of aconductor 35 to the source of plate supply potential. This source isconnected also by a conductor 36 to the screen-grid of the tube 2. Thetuning coil 34 is tuned to the same frequency as the coil 2i) by atuning condenser 3l and the Q of the tuned circuit is reduced by aresistance 38 connected in parallel with the coil 3S. The plate of thetube 2 and the plate conductor33 are coupled to the screen grid of thetube I through a coupling condenser 39.

In the circuit just described, the resistances I3, I'I and I8 are chosento bias the tube l at or near cut-olf. The feed-back connection betweenthe plate of the tube 2 and the screen grid of the tube I and betweenthe plate of the tube I and the control grid of the tube 2 provides aregenerative circuit which may oscillate in the absence of an inputsignal. Such oscillation is not, however, a requisite for properoperation.

Assuming, for example, that the input frequency which is applied to theinput terminals 3 and d is say 60 kc. and it is desired to produceacross the output terminals 25 and 26 a frequency of 2U kilocycles, thecircuit 34-31' will be tuned to a frequency ofy 20 kilocycles. Theresistances I3, II and I8 are also so proportioned as to apply to thescreen grid of the tuoe Iv a positive direct potential which is less,than the output voltage developed by the regulating tube 2 so that foreach positive half cycle of the output of the tube 2, tne potentialofthe screen grid of the tube I will be raised to a substantial value,whereas during the negative half cycles of the output of tne tube 2, thescreen grid of the tube I will be driven negative. The resistance Ilahas a high ohmic value and operates as a grid leak to hold .the screengrid negative during operation exceptv during the positive maxima in theoutputs from tube 2. Since the tube I is operative only when the screengrid has a positive potential impressed thereon, it is seen that thetube I Yoperates as an electronic switch which is closed only once foreach cycle of the output of the oscillator 2 or, inthe assumed example,at a frequency of 20 kilocycles.

Since the input to the control grid of the tube I, in the assumedexample, is at a frequency of 60 kilocycles, two positive grid swingsoccur while the tube is blocked and one occurs while the tube isconductive.r There is thus applied to the pri-V mary winding 2i) of thetransformer 2| a 20 kilocycle voltage.' The current which flows inconductor 22 is not of sine wave foi-m but is rich in harmonics. This isadvantageous since it permits exciting frequency multiplying circuits ifit is required to produce an end frequency different than the outputfrequency, as for example 40 kc.

The use of low Q tuned circuits for the circuits 20, 23 and 34, 31 makesthe tuning very broad. VSince the tube 2 is excited at its con- 4 trolgrid by the output of the tube I, the Voutput of tube 2 is in forcedsub-multiple synchronism With the input signal which is applied to thegrid of the tube. I. This is accomplished even though the optimum tuningof the tuned circuits 2t, 23 and 34, 31 is not precisely one-third inputfrequency.

In Fig. 2 I have illustrated a circuit which is very similar to thatshown in Fig. l but which is intended particularly for use at audiofrequency. The frequency which is to be divided is applied to inputterminals 50 and 5I, the latter being connected to ground as by aconductor 52. The terminal 53 is connected to the grid of a vacuum' tube53 through a coupling condenser 54. The vacuum tube 5.3 and a secondvacuum tube 55 are shown in Fig. 2 as being pentodes. However, thetetrode or screen grid type of tube may be em"- ployed if desired. Thecathode, screen grid and suppressor grid circuits for the tube 53 areidentical to those shown for the tube l in Fig. 1 and the values of theresistances involved are chosen to produce the same type of operation ashas been described in connection with Fig. l.

The plate of the tube 53 is coupled to the grid of the tube 55 through acoupling `condenser 55. A plate load resistance 5l by-passed by a bypasscondenser 58 serves as a plate load for the tube 53 and is connectedbetween the plate and a suitable source of plate potential. The grid ofthe tube 55 is connected to ground through a grid resistance 59. Thecathode and suppressor grids of the tube 55 are connected to groundthrough a cathode-biasing resistor 6I which is preferably by-passed by acondenser 62. The plate of the tube 55 is coupled through a couplingcondenser 63 to one of a pair of output terminals 54 and 55, the otherof which is connected to ground as by a conductor e5. The plate load forthe tube 55 comprises a tuned circuit including a tuning coil orinductance El shunted by a tuning condenser 8 and loading resistance 69.The screen grid and the plate of the tube 55 derive operating potentialthrough a conductor 'lil connected to the source of plate supplypotential. The plate of the tube 55 is coupled in a feed-back circuitincluding a coupling condenser il to the screen grid of the tube 53.

The operation of the circuit shown in Fig. 2 is nearly identical withthat which has been described in connection with Figli. The tubes `5 3and 55 constitute a circuit which may oscillate at the desired outputfrequency in the absence of an input signal. The operation of the tube53 is periodically blocked at the output frequency by the application ofnegative voltages to the screen grid of the tube 53 in the same manneras has been described in Fig. l. The principal difference between thetwo circuits is, rst, that the circuit shown in Fig. 2 has been arrangedfor operation at low or audible frequencies, whereas the circuit of Fig.l is intended to operate at relatively high o1' radio frequencies secondthat the resistance capacity network 55- 53 operates as a low Q bandpassfilter which, through the proper selection of resistance and capacityvalues, may be in elfect tuned to the selected sub-multiple of the inputfrequency.

From the foregoing, it will be observed that I have provided frequencydivider circuits which may be used at either radio or audio frequencyand that these circuits serve to produce an output frequency to whichthe input frequency bears a whole number ratio. These circuits include asynchronizing tie which maintains this whole number ratio between theinput and output frequencies.

It will also be noted that tuned circuits are employed for determiningthe output frequency and that for this reason the circuits are notsubject to the frequency error which characterizes multi-vibratorcircuits, depending for frequency regulation upon the time constant ofresistance capacity circuits. With circuits of the character justdescribed, input voltages may be varied from one-half the normaloperating voltage to double the operating voltage without affecting theoutput frequency. This is a particular advantage over the previouslyemployed multi-vibrator circuits in which the output frequency was afunction of the input voltage, requiring variations in input voltage tobe held below ten percent.

I have found that frequency divider circuits of the character describedherein will permit operation at an input frequency to output frequencyratio as high as twelve, although more satisfactory operation isobtained at lower ratios.

While I have shown and described the preferred embodiment of myinvention, I do not desire to be limited to any of the details ofconstruction shown or described herein, except as defined in theappended claims.

Iclaim:

1. In a frequency dividing circuit, the combination of: a vacuum tubeamplifier stage including an input circuit tuned to an input frequencyto be divided; a vacuum tube regulating stage including an outputycircuit tuned to a sub-multiple of said input frequency; a couplingcircuit also tuned to said sub-multiple of said input frequency andcoupling the output of said arnplifier stage to the input of saidregulating stage;

and means coupling said regulating stage to said amplifier stage forrendering said amplifier stage conductive only during coincidence intime of positive maxima of said input frequency and said sub-multiplethereof.

2. In a frequency divider for producing an output frequency to which aninput frequency bears a whole number ratio, the combination of: a vacuumtube connected in a regenerative circuit and coupled to band-pass filtercircuits all resonant at said output frequency; and a synchronizingcircuit energized at said input frequency and connected to force saidcircuit to oscillate in submultiple synchronism with said inputfrequency.

3. In a frequency divider for producing an output frequency to which aninput frequency bears a whole number ratio, the combination of: a vacuumtube amplifier circuit including an input circuit to be supplied withsaid input frequency and including an output circuit tuned to saidoutput frequency; a vacuum tube regulating stage tuned to said outputfrequency; means coupling said regulating stage to said output circuitfor exciting said regulating stage at the frequency of said outputcircuit; and a control circuit connecting said regulating stage to saidamplifier circuit for alternately blocking and releasing said amplifiercircuit at the frequency of said output circuit.

4. In a frequency divider for producing an output frequency to which aninput frequency bears a whole number ratio, the combination of: an

amplifier circuit comprising a vacuum tube having a pair of controlelements and a plate and comprising also input and output circuitsincluding, respectively, one of said control elements and said plate;means for energizing said input circuit at said input frequency; avacuum tube regulating stage excited from said output circuit and tunedto said output frequency; and a control circuit connecting saidregulating stage to the other of said control elements for renderingsaid amplifier circuit conductive only when the positive half cycle ofthe regulating stage output coincides in time with the positive halfcycle of said input frequency.

5. In a frequency divider for producing an output frequency to which aninput frequency bears a whole number ratio, the combination of: anamplifier circuit comprising a vacuum tube having a pair of controlelements and a plate and comprising also input and output circuitsincluding, respectively, one of said control elements and said plate;means normally biasing said vacuum tube substantially at cut-off; meansfor energizing said input circuit at said input frequency; a vacuum tuberegulating stage excited from said output circuit and broadly tuned tosaid output frequency; and a control circuit connecting said regulatingstage to said other control element for holding said element at anegative potential except during the peak of the positive half cycle ofsaid regulating stage output.

6. In a frequency divider for producing an output frequency to which aninput frequency bears a whole number ratio, the combination of: a vacuumtube amplier stage including an output circuit and two input circuits; avacuum tube regulating stage connected in a regenerative circuit betweensaid output circuit and one of said input circuits; means tuning saidregenerative circuit to said output frequency; and means for applyingsaid input frequency to the other of said input circuits.

7. In a frequency divider for producing an output frequency to which aninput frequency bears a whole number ratio, the combination of: an.V

input circuit to be supplied with current of said input frequency; anoutput circuit for supplying said output frequency; a vacuum tubeconnected in an electronic switch circuit between said input and outputcircuits and including a control element for opening and closing saidswitch; a vacuum tube regulating stage; a synchronizing circuitconnecting said regulating stage between said output circuit and saidcontrol element; and means tuning said synchronizing circuit to saidoutput frequency.

WILLIAM J. OBRIEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,874,845 Albersheim Aug. 3U,1932 2,124,191 Geiger July 19, 1938 2,344,678 Crosby Mar. 21, 1944FOREIGN PATENTS Number Country Date 479,935 Great Britain Feb. 14, 1938

